How do I convert a Go array of strings to a C array of strings?

Question

I am using cgo in a project, and I want to export a function for use. Here's an example of what I want to achieve:

package csplit

import (
    "C"
    "strings"
)

//export Split
/* The Split function takes two C strings, the second of which represents
   a substring to split on, and returns an array of strings. Example:
       Split("1,2", ",") // gives ["1", "2"]
*/
func Split(original *C.char, split *C.char) []*C.char {
        goResult := strings.Split(C.GoString(original), C.GoString(split))
        cResult := make([]*C.char, len(goResult))

        for idx, substring := range goResult {
                cResult[idx] = C.CString(substring)
        }

        return cResult
}

The problem is that the return type is Go allocated data, and not moved into the C heap. This panics with: runtime error: cgo result has Go pointer

Mr_Pink · Accepted Answer · 2024-11-18 13:56:24Z

15

You're returning a Go slice which is allocated in Go, and is a different structure than a C array. You need to allocate an array in C:

//export Split
func Split(original *C.char, split *C.char) **C.char {
    goResult := strings.Split(C.GoString(original), C.GoString(split))
    cArray := C.malloc(C.size_t(len(goResult)) * C.size_t(unsafe.Sizeof(uintptr(0))))

    // convert the C array to a Go Array so we can index it
    a := unsafe.Slice((**C.char)(cArray), len(goResult))

    for idx, substring := range goResult {
        a[idx] = C.CString(substring)
    }

    return (**C.char)(cArray)
}

edited Nov 18, 2024 at 13:56

answered Jan 5, 2017 at 19:32

Mr_Pink

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6 Comments

1ijk Over a year ago

ah, thank you. I believe that allocating the C array first may be the trick. (will accept once I test it out for my use case, or add more details to the question if necessary)

Mr_Pink Over a year ago

@haiqus: sorry, I made a mistake typing this out, and should have included a sizeof in the malloc (I was thinking char instead of char* which would have been obvious had I not omitted the sizeof). I revised it to show the proper malloc size.

fbiville Over a year ago

I'm late to the party but what does (*[1<<30 - 1]*C.char) mean?

Mr_Pink Over a year ago

@fbiville: stackoverflow.com/questions/48756732/…

Mr_Pink Over a year ago

@MikeSchinkel, yes now you would use unsafe.Slice.

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